Papillomaviruses (PV) are non-enveloped DNA viruses that induce hyperproliferative lesions of the epithelia. The papillomaviruses are widespread in nature and have been recognized in higher vertebrates. Viruses have been characterized, amongst others, from humans, cattle, rabbits, horses, and dogs. The first papillomavirus was described in 1933 as cottontail rabbit papillomavirus (CRPV). Since then, the cottontail rabbit as well as bovine papillomavirus type 1 (BPV-1) have served as experimental prototypes for studies on papillomaviruses. Most animal papillomaviruses are associated with purely epithelial proliferative lesions, and most lesions in animals are cutaneous. In the human more than 100 types of papillomavirus (HPV) have been identified and they have been catalogued by site of infection: cutaneous epithelium and mucosal epithelium (oral and genital mucosa). The cutaneous-related diseases include flat warts, plantar warts, etc. The mucosal-related diseases include laryngeal papillomas and anogenital diseases comprising cervical carcinomas (Fields, 1996, Virology, 3rd ed. Lippincott—Raven Pub., Philadelphia, N.Y.; Bernard, H-U., 2005. J. Clin. Virol. 328: S1-S6).
Human papillomavirus (HPV) is one of the most prevalent sexually transmitted infections in the world. The majority of HPV infections are harmless. Some types of HPV cause genital warts, which appear as single or multiple bumps in the genital areas of men and women including the vagina, cervix, vulva (area outside of the vagina), penis, and rectum. Many people infected with HPV have no symptoms.
While most HPV subtypes result in benign lesions, certain subtypes are considered high-risk and can lead to more serious lesions, such as cervical and anal dysplasia. Fifteen HPV types were recently classified as high-risk types (Munoz, N. et al. 2003. N. Engl. J. Med. 348(6):518-27.) These high-risk subtypes are genetically diverse, demonstrating >10% sequence divergence at the L1 gene, a major virus capsid protein. (Bernard, H-U., 2005. J. Clin. Virol. 328: S1-S6).
Women having HPV infection are often asymptomatic and may only discover their lesion after cervical screening. Cervical screening is widely performed using the Pap test. A Pap test is a histological evaluation of cervical tissue which is used to identify abnormal cervical cells. As part of a Pap test, the presence of HPV infection and the specific subtype may be determined with the use of nucleic acid based assays such as PCR or the commercial Hybrid Capture II technique (HCII) (Digene, Gaithersburg, Md., U.S.A).
Abnormal cervical cells, if identified, are graded as LSIL (low-grade-squamous intraepithelial lesions) having a low risk of progressing to cancer (including CIN-1 designated cells (“cervical intraepithelial neoplasia-1”)); or HSIL (High-grade squamous intraepithelial lesions), including CIN-2 and CIN-3 designated cells, having a higher likelihood of progressing to cancer.
About 85% of low-grade lesions spontaneously regress, and the remainder either stay unchanged, or progress to high-grade lesions. About 10% of high-grade lesions, if left untreated, are expected to transform into cancerous tissues. HPV-16 and HPV-18 are most often associated with dysplasias, although several other transforming HPV subtypes are also associated with dysplasias.
Recent studies indicate that up to 89% of HIV positive homosexual males may be infected with these high-risk subtypes of HPV. HIV positive patients are also more likely to be infected with multiple subtypes of HPV at the same time, which is associated with a higher risk of dysplasia progression.
Evidence over the last two decades has led to a broad acceptance that HPV infection is necessary, though not sufficient, for the development of cervical cancer. The presence of HPV in cervical cancer is estimated at 99.7%. Anal cancer is thought to have a similar association between HPV infection and the development of anal dysplasia and anal cancer as is the case with cervical cancer. In one study of HIV negative patients with anal cancer, HPV infection was found in 88% of anal cancers. In the US in 2003, 12,200 new cases of cervical cancer and 4, 100 cervical-cancer deaths were predicted along with 4,000 new cases of anal cancer and 500 anal-cancer deaths. While the incidence of cervical cancer has decreased in the last four decades due to widespread preventive screening, the incidence of anal cancer is increasing. The increase in anal cancer incidence may be attributed in part to HIV infection since HIV positive patients have a higher incidence of anal cancer than the general population. While anal cancer has an incidence of 0.9 cases per 100,000 in the general population, anal cancer has an incidence of 35 cases per 100,000 in the homosexual male population and 70-100 cases per 100,000 in the HIV positive homosexual male population. In fact, due to the high prevalence of anal dysplasia among HIV-infected patients and a growing trend of anal cancers, the 2003 USPHA/IDSA Guidelines for the Treatment of Opportunistic Infections in HIV Positive Patients will include treatment guidelines for patients diagnosed with anal dysplasia.
There is no known cure for HPV infection. There are treatments for genital warts, although they often disappear even without treatment. The method of treatment depends on factors such as the size and location of the genital warts. Among the treatments used are Imiquimod cream, 20 percent podophyllin antimitotic solution, 0.5 percent podofilox solution, 5 percent 5-fluorouracil cream, and Trichloroacetic acid. The use of podophyllin or podofilox is not recommended for pregnant women because they are absorbed by the skin and may cause birth defects. The use of 5-fluorouracil cream is also not recommended for pregnant women. Small genital warts can be physically removed by freezing (cryosurgery), burning (electrocautery) or laser treatment. Large warts that do not responded to other treatment may have to be removed by surgery. Genital warts have been known to return following physical removal; in these instances α-interferon has been directly injected into these warts. However, α-interferon is expensive, and its use does not reduce the rate of return of the genital warts.
As such there exists an unmet need for effective HPV treatment. Surprisingly, compounds have been discovered that meet this need, and provide other benefits as well.
Recently, double-stranded RNA molecules (dsRNA) have been shown to block gene expression in a highly conserved regulatory mechanism known as RNA interference (RNAi). WO 99/32619 (Fire et al.) discloses the use of a dsRNA of at least 25 nucleotides in length to inhibit the expression of genes in C. elegans. dsRNA has also been shown to degrade target RNA in other organisms, including plants (see, e.g., WO 99/53050, Waterhouse et al.; and WO 99/61631, Heifetz et al.), Drosophila (see, e.g., Yang, D., et al., Curr. Biol. (2000) 10:1191-1200), and mammals (see WO 00/44895, Limmer; and DE 101 00 586.5, Kreutzer et al.). This natural mechanism has now become the focus for the development of a new class of pharmaceutical agents for treating disorders that are caused by the aberrant or unwanted regulation of a gene.
PCT Publication WO 03/008573 discloses a previous effort to develop a nucleic acid based medicament for the treatment of disease caused by HPV infection. This publication reports the use of two siRNAs directed to HPV mRNA to inhibit HPV replication in a cell based system; a related publication is found at Jiang, M. et al. 2005. N. A. R. 33(18): e151.
Despite significant advances in the field of RNAi and advances in the treatment of pathological processes mediated by HPV infection, there remains a need for agents that can inhibit the progression of HPV infection and that can treat diseases associated with HPV infection. The challenge is exacerbated because such agents must be designed to inhibit all the high-risk HPV subtypes, which together display a wide degree of genotypic diversity.